Multiport and differential Vector Network Analyzer (VNA) measurements are used in a wide variety of communications applications. The purpose of a vector network analyzer is to measure the magnitude and phase of the reflection and transmission characteristics of a microwave component as functions of frequency. A component can be inserted between the test ports, the test signal is rapidly tuned over a span of frequency, and portions of this signal are reflected from and transmitted through the component. These measurements are critical for accurately analyzing the characteristics of equipment such as telecommunications and data components, gigabit Ethernet backplanes, and other devices. However, as multiport measurements are attempted at higher frequencies on these devices, the process of maintaining accuracy becomes more difficult.
One approach for performing these measurements consists of placing a six switch fabric (arrangement of switches), for multiplexing in front of a receiver and source matrix, which can be part of a 2-port VNA. In this case a series of 2 port calibrations are renormalized against the off-state impedances of the various switches before combining their signals. For lower frequencies this approach works well, but for higher frequencies (above 50 GHz) the insertion loss becomes intolerably high (greater than 10–15 dB at 60 GHz). In an alternate approach the test couplers are moved out to the test ports, but a similar switch fabric is used. However, such a system can also have similar insertion loss problems and be prohibitively expensive. Simplified switch fabrics have been identified but major changes are needed to traditional calibration techniques to ensure measurement accuracy when using such fabrics. What is needed is an improved calibration system that can enable the use of a switch fabric design that has low insertion loss, is easy to construct.